Standard color photographic negative film that is widely used in still cameras today is designed and manufactured to contain three superimposed, semi-independent color sensing layers. Spectral sensitivity curves for photographic negative film show the typical response of the three layers of photographic film over the visible light spectrum; assuming equal radiated power at each wavelength. In particular, it is known that the top layer responds primarily to light of short wavelength (blue light), the middle layer responds primarily to light of medium wavelength (green light) and the bottom layer responds to light of long wavelength (red light). When film with these types of spectral sensitivities is exposed to visible light, each spot on the film records the amount of blue, green and red light, or flux. Incident flux creates what is referred to as the latent image.
In conventional color photographic development systems, the exposed film is chemically processed to produce dyes in the three layers with color densities directly proportional to the blue, green, and red spectral exposures that were recorded in the latent image. Yellow dye is produced in the top layer, magenta dye in the middle layer, and cyan dye in the bottom layer. Through a separate conventional process, positive photographic images may then be electronically scanned to produce a digital image.
Conventional electronic scanning of developed photographic negative film to produce digital images is done by passing visible light through the developed negative and using filters with appropriate spectral responsivities to detect, at each location on the film, the densities of the yellow, magenta and cyan dyes in the photographic negative. The density values detected in this way are indirect measures of the blue, green and red light that initially exposed each location on the film. These measured density values constitute three values used as the blue, green and red values for each corresponding location, or pixel, in the digital image. Further processing of these pixel values is often performed to produce a digital image that accurately reproduces the original scene and that is pleasing to the human eye.
Image enhancement has been the subject of a large body of film processing technology. A common feature of all digital film processing technology is that the film to be scanned must be relatively flat during the optical scan. Furthermore, the optical scan best occurs using a relatively uniform velocity during the scan period. Small imperfections in the film, such as tearing, creases, scratches, foreign objects and fluids decrease the efficacy of the digital scan. Large imperfections make digital film processing and conventional scanning very difficult.
Large imperfections to the film surface, such as broken, ripped or torn sprocket holes, are encountered frequently during automated film processing. In film processing using chemical development tanks, tears to the sprocket holes are generally not an issue because they are not used to transfer the film from tank to tank. For example, torn sprocket holes occur when the user, or in the case of automated cameras, the auto-drive advances the film too far, breaking one or more of the sprocket holes.
In addition to large imperfections, such as sprocket hole breakage, other imperfections may occur when foreign objects, such as water, particles (e.g., dust), and oils contact the film. Exposure to these foreign objects may even occur while the film is still in its original canister. Creases in the film are yet another imperfection that may occur when the film is reverse-wound.